Parabolic trough collector solar plants normally use a heat transfer fluid that can be both thermal oil and water, in direct steam generation collectors. In the case of collectors that use oil as a heat transfer fluid, a thermal oil is heated in the collectors which, after passing through a heat exchanger, allows the production of pressurized steam that is subsequently expanded in a conventional Rankine cycle turbine.
There are a large variety of compounds which are used as thermal oil for solar plants, each having specific characteristics. Specifically, one of these oils is composed of a eutectic mixture of biphenyl and diphenyl oxide (marketed under the name Dowtherm A or Therminol VP1). The properties of this type of oil make it especially suited for heat transfer within a temperature range of up to 400° C.
The thermal stability of a fluid is determined by its composition. When the temperature of the oil reaches the working values of the solar thermal plant (temperatures of up to 400° C.), the molecular bonds of the fluid structure break to form two large types of degradation products: light compounds and heavy compounds.
In the specific case of the aforementioned oil, it undergoes slow decomposition into light components (low boilers), essentially benzene and phenol, and heavy components (high boilers), essentially o-terphenyl, m-terphenyl, p-terphenyl and 2-phenoxybiphenyl. The concentration of these oil degeneration products should not exceed certain limits due to the fact that light components, having a high vapour pressure, increase the pressure in the system and can cause cavitation in the pumps, while the heavy components limit the efficiency of the heat transfer as, among other things, they reduce the specific heat of the heat transfer oil and increase its viscosity.
In order to avoid accumulation, said heavy components and light components must be periodically removed from the facility, as they modify the properties of the heat transfer oil.
It has been experimentally verified that simple flash distillation (single-step distillation) followed by partial condensation of the vapour current generated in the distiller is unable to effectively regenerate the oil. The separation of significant amounts of heavy components is necessarily associated with considerable oil losses. Thus, for example, the elimination of 70% of the heavy components entails oil losses of approximately 30%.
The technical problem being addressed consists of describing a thermal oil regeneration plant capable of minimising oil degradation and oil losses and improving regeneration performance, as the price of said oil is considerably high.